1. Field of the Invention
The present invention relates to an image formation apparatus for forming a color dot image in response to an input image signal.
2. Description of the Prior Art
In an apparatus of the type described above, when a color image is formed, three R, G and B input color signals supplied in units of pixels are subjected to image processing to form three Y, M and C output color signals. The dot density per unit area of an image or the dot size is controlled in accordance with the level of each output color signal to form a color image including a halftone portion.
One technique of controlling the dot density per unit area of an image is generally referred to as density modulation. The density modulation technique includes a density pattern method and a dither method. According to the density pattern method, a unit block of an output image is divided into a predetermined number of cells, and the number of ON (colored) or OFF (non-colored) cells is varied in accordance with the density of each pixel of the original image. In the dither method, the density of each pixel of the original image is compared with a threshold value determined under prescribed conditions, and the ON/OFF state of each output dot is controlled according to the comparison result. A plurality of threshold levels are used or the input pixel is level-shifted to provide a gray level or a half tone. In the density modulation method, the dot size remains the same. However, when the number of cells within a single pixel is increased in order to obtain a certain gray level range, one pixel becomes large and resolution is lowered.
In contrast, in an analog technique, in which the dot size is changed, a single dot can express a gray level and resolution can be improved. On the other hand, since the modulating width of the dot size is limited, the gray level range is limited. In view of this problem, a method has recently been proposed in which dots having a plurality of inherent densities are used (for example, by using inks of different dye concentrations) for one color. This method is described in, for example, U.S. patent application Ser. No. 491, 654 filed on May 5, 1983.
This analog method of utilizing dots of different densities provides an excellent halftone effect and resolution and allows excellent reproduction of relatively bright colors such as skin color. The method is therefore very effective in the field of pictorial printing such as portraits and scenic views. However, in the case of artificial images such as computer graphics, figures or graphs wherein predetermined areas are printed in the same color, when readout values are obtained for an image portion within a single area due to shading or noise effect, an area printed with dark dots and an area printed with bright dots may be present at the same time, thereby forming a pseudo edge line at the boundary between the two areas, which results in poor printing.